1. Field of the Invention
The present invention relates to a laser module and a method of manufacture therefor. Particularly, the present invention relates to a laser module in which constituent parts, including a semiconductor laser that emits light within a wavelength range of 350 to 450 nm, are hermetically sealed, and a method of manufacture therefor.
2. Description of the Related Art
Conventional optical modules that irradiate or generate ultraviolet rays having wavelengths of 400 nm or less are known. These optical modules have a problem in that optical loss occurs in the optical components that constitute the optical modules, due to the ultraviolet rays irradiated or generated thereby. The optical loss is thought to occur due to water and oil within the atmosphere being decomposed by the ultraviolet rays, then accumulating on the surfaces of the optical components.
U.S. Pat. No. 6,404,786 discloses an ultraviolet ray irradiating optical system that prevents decreases in output of ultraviolet laser light. The ultraviolet ray irradiating optical system seals optical components within an atmosphere comprising nitrogen at a purity of 99.9% or greater, dried air at a purity of 99.9% or greater, a gas containing water at 0.1% or less, or a gas containing hydrocarbons at 0.1% or less. Thereby, the accumulation of decomposed matter on the optical components, and therefore, decreases in output of ultraviolet light, is prevented.
The present inventors have analyzed the sealed atmospheres of modules that include semiconductor lasers that emit light within a wavelength range of 350 to 450 nm. As a result, it has been found that various compounds are included in the sealed atmospheres, as will be described below. In addition, it has been found that specific organic gas components, generated by solid organic matter which is attached to optical components and mechanical components employed in the modules, are the main causes of deterioration in laser properties.
In conventional laser modules, organic adhesives, such as that disclosed in Japanese Unexamined Patent Publication No. 2001-177166 and epoxy based adhesives such as NOA61 by Noland Co. are employed to fix laser elements and optical systems. Solid organic matter, which is attached to the optical and mechanical components within the modules are introduced from the module production atmosphere. Therefore, organic residue is present, even if cleansing is performed. Organic gas is generated from these solid organic materials, and a certain amount of the gas (so-called “outgas”) fills the interior of the sealed module. Further, there are cases in which the outgas contains compounds that include silicon atoms, phosphor atoms, sulfur atoms and the like, depending on the type of organic matter.
Solvents such as isopropyl alcohol (molecular weight: 60.10, boiling point: 82.4° C.) and acetone (molecular weight: 58.08, boiling point: 56.1 to 56.5° C.) are utilized to cleanse parts that constitute the interior of the modules in the sealing step. Organic gas components having low molecular weights and low boiling points are generated by the solvents, and introduced as impurities into the sealing gas, such as dried nitrogen and dried air.
For this reason, organic gas components (hereinafter referred to as “outgas components”), which are generated from the solid organic materials, and organic gas components (hereinafter referred to as “impurity components”), which are introduced during the sealing step, are both present within the sealed atmosphere. As a result of analysis by gas chromatography, illustrated in FIG. 5 and FIG. 6, it can be seen that the distributions of molecular weights and boiling points clearly differ between the two types of components.
FIG. 5 is a graph, in which the volumes of the impurity components and the outgas components are plotted against the molecular weights thereof. The total amount of each of the components, as detected by a GC-MASS (Gas Chromatography/Mass Spectrometer), is designated as 100% in the graph of FIG. 5. FIG. 6 is a graph, in which the volumes of the impurity components and the outgas components are plotted against the boiling points thereof. The total amount of each of the components, as detected by a GC-MASS, is designated as 100% in the graph of FIG. 6 as well.
That is, the molecular weights of the outgas components are distributed within a range greater than or equal to 70, whereas the molecular weights of the impurity components are distributed within a range less than 70. In addition, the boiling points of the outgas components are distributed within a range greater than or equal to 70° C., whereas the boiling points of the impurity components are distributed within a range less than 100° C.
Next, the relationships between the concentration of the two types of organic gases in the sealed atmosphere and the deterioration rates of laser modules were investigated. Note that the laser module employed in the investigation is of the same construction as those which are to be described later, with reference to FIG. 1 through FIG. 4, except for the fact that organic adhesives were utilized therein. The results of the investigation are illustrated in FIG. 7. The deterioration rates are indicated by the amount of increased drive current necessary per hour to drive all elements of the laser modules in the case that each light emitting point thereof are driven at 100 mW.
In the graph of FIG. 7, plotted points ♦ indicate the relationship between the concentration of outgas components and the deterioration rate of the module. Plotted points □ indicate the relationship between the concentration of impurity components and the deterioration rate of the module. The concentrations of the impurity components were adjusted by artificially manipulating the concentration of acetone within the sealed atmosphere.
As can be seen from FIG. 7, the rate of increase in drive current rises drastically when the concentration of the outgas components becomes 1000 ppm or greater, conspicuously accelerating deterioration of the module. The cause of the accelerated deterioration is assumed to be the accumulation of solid matter, generated by photolysis of the outgas components, on light emitting portions and optical components within the module.
On the other hand, no accumulation of solid matter on the light emitting portions or the optical components within the module is observed, even when the concentration of the impurity components becomes 1000 ppm or greater. This is because the impurity components do not solidify at room temperature even if they are photolyzed. Note that even if the impurity component is changed from acetone to isopropyl alcohol, no accumulation of solid matter is observed.